In galvanizing plants, a vessel for cleaning many forms of steel is charged with an acid. Normally, the acid has a pH of about 1. Typically a strong acid is used and the most common acid is sulfuric acid. The vessel of container is heated to something less than boiling. Elevation of the temperatures elevates the chemical activity occuring in the tank. Prior to galvanization, the steel to be coated with zinc must be first cleaned. The steel must be cleaned of rust, dirt, scale, and other materials. After cleaning, the galvanizing process can then proceed. The zinc coating adheres successfully on a clean surface. For this reason, pre-cleaning of the steel must occur before galvanization.
Sometimes, the galvanization process will not be carried on perfectly and as a consequence, the partially galvanized steel must be cleaned. Thus it is necessary to return the incorrectly galvanized steel to the cleaning tank where it will be stripped of the zinc coating thereon.
After some period of time, the acid cleaning tank becomes heavily laden with ions of metal molecules. This of course reduces the effectiveness of the acid bath. At a point in time when the acid bath is substantially contaminated, it is no longer able to clean the steel. It has been heretofore necessary to dump all of the acid from the cleaning tank and replace it with a fresh batch of acid. However, in recent years with concern about the environment rising, it has become exceedingly difficult to dispose of several thousand gallons of spent acid. While the acid may not be usable commercially at this juncture it remains a strong acid and most locales forbid the dumping of a strong acid laden with metal ions into the sewer system. One alternative is to truck the spent or contaminated acid away to an authorized disposal facility where it might be combined with available waste bases for neutralization of the bases. As can be understood, disposal techniques are quite expensive.
The present invention provides a method whereby the spent or contaminated acid can be purified which enables it to be re-used. A tremendous savings in cost is achieved by avoiding disposal problems of the spent acid. An ecological advantage is also achieved by avoiding the pumping of the spent acid into sewers.
The present invention thus purifies and recovers the acid to be re-used indefinitely by cutting across interdisciplanary lines to take advantage of techniques from electro-chemistry, electro-dialysis, and more traditional wet chemistry which disciplines cannot in and of themselves solve this problem. This is done at the cost of electric power consumption. However, this cost is more than recovered by the recovery of purified acid which can be returned to the cleaning tanks. There is an additional benefit. The acid is used as mentioned above to clean metal plates and eventually becomes heavily laden with various metals. This method enables recovery of the metals which then have resale value primarily but not exclusively in the agricultural chemical market. This achievement is noteworthy in that the recovery of purified acid and metal from the process more than covers the cost of operation.